DefaultedDictionary.swift

struct DefaultedDictionary<Key: Hashable, Value> {
    var dict: [Key:Value]
    let fallback: Value

    subscript(key: Key) -> Value {
        get {
            return dict[key] ?? fallback
        }
        set {
            dict[key] = newValue
        }
    }
}

extension Dictionary {
    func defaultingTo(value: Value) -> DefaultedDictionary<Key, Value> {
        return DefaultedDictionary(dict: self, fallback: value)
    }
}